1. Field of the Invention
The present invention relates to an improved process for producing .epsilon.-caprolactam which comprises subjecting cyclohexanone oxime to Beckmann rearrangement under gaseous phase reaction conditions in a fluidized bed reaction using a solid catalyst, wherein a fluidized bed reaction system with a re-generator for the catalyst is used.
2. Description of Related Art
.epsilon.-Caprolactam is a key chemical product used as a raw material for Nylon or the like. Among processes for its production, a process that has conventionally been adopted in the industrial production thereof is a process in which cyclohexanone oxime is subjected to Beckmann rearrangement with fuming sulfuric acid under liquid phase conditions.
In addition, many variations for its production have been proposed in which cyclohexanone oxime is subjected to Beckmann rearrangement with a solid acid as the catalyst under gaseous phase reaction conditions (gaseous phase Beckmann rearrangement). For example, processes are known using a boric acid catalyst (JP-A-53-37686, JP-A-46-12125), a silica-alumina catalyst (British Patent No. 881,927), a solid phosphoric acid catalyst (British Patent No. 881,956), a Y-type zeolite catalyst (Journal of Catalysis, 6, 247 (1966)), a crystalline aluminosilicate catalyst (JP-A-57-139062) or the like.
Furthermore, a process has been proposed in which a lower alcohol coexists in the reaction system (JP-A-2-275850).
As the reaction methodology, a method has been proposed in which cyclohexanone oxime is subjected to rearrangement in a fluidized bed using a boric acid catalyst on a carrier under gaseous phase conditions, an oxygen-containing gas is concurrently utilized to re-generate the catalyst in the fluidized bed, and boron trioxide or boric acid is added to the catalyst in fluidized state before the catalyst is returned to the reactor for the rearrangement (JP-A-55-53267).
Yet further, in carrying out gaseous phase Beckmann rearrangement with a boric acid catalyst carried on carbon in a fluidized bed system, the difference between the organic nitrogen content of the catalyst in a reactor and that of the catalyst in a re-generator is maintained within a certain range when the catalyst is withdrawn from the reactor of the fluidized bed to re-generate and returned upon re-generation to the bed (JP-A-53-35691). In this case, it is known that a scattering of a part of boric acid as a component of the catalyst is unavoidable in the re-generation step, and almost the same amount of carrier carbon for the catalyst as that of scattering boric acid is lost by burning.
The above-described conventional Beckmann rearrangement process using fuming sulfuric acid, which has been widely adopted in the industry, had not only a problem in that a large amount of fuming sulfuric acid is required but also a problem in that neutralization of sulfuric acid with ammonia is necessary for recovering .epsilon.-caprolactam from the reaction product of Beckmann rearrangement. In this step, as much as about 1.5 ton of ammonium sulfate per ton .epsilon.-caprolactam is produced as the by-product.
On the other hand, the gaseous phase Beckmann rearrangement process using a solid catalyst has an advantage that no ammonium sulfate is produced in the step of the Beckmann rearrangement thereof. Various catalysts suitable for the gaseous phase Beckmann rearrangement have been proposed. All the catalysts have a problem that carbonaceous substances deposit on the catalyst during the reaction, resulting in coverage of active sites by the carbonaceous substances, which leads to gradual deactivation of the catalyst. Recovery of the catalytic activity is possible by interrupting the reaction at an appropriate time, sending an oxygen-containing gas to the catalyst bed and removing the carbonaceous substances by oxidation. However, this raises another problem that the production of .epsilon.-caprolactam is interrupted during the regeneration operation for the catalyst in the case of the fixed bed reaction system, and a switching operation between the reaction and the re-generation is troublesome. A method is also known in which both a step of gaseous phase Beckmann rearrangement reaction and a re-generation reaction step for the catalyst are performed in a fluidized bed system, and both the rearrangement reaction and the re-generation are continuously carried out by circulating the catalyst through a reactor and a re-generator. In this case, there is a problem that it is difficult to maintain a high conversion or a high selectivity in the Beckmann rearrangement reaction and to continue a stable production for a long term and, therefore, an improved process which solves such a problem has been demanded.